In recent years, an optical image measurement technology of forming an image of the surface or inside of a measured object by using a laser light source or the like has gained attention. The optical image measurement technology is expected to be applied particularly in the medical field because this technology is noninvasive to a human body unlike the conventional X-ray CT technology.
An example of typical methods relating to the optical image measurement technology is the OCT technology (referred to as the “optical coherence tomographic imaging method” or the like). This technology makes it possible to detect a reflected light or a transmitted light by a measured object with excellent distance resolution in the μm order by utilizing the low coherency of a broadband light source having a broad spectrum width, such as a Super Luminescent Diode (SLD) (refer to Non-Patent Document 1, for example).
As an example of a device utilizing the OCT technology, a conventional optical image measurement device using a Michelson-type interferometer will be described. The basic configuration of the optical image measurement device is shown in FIG. 10. An optical image measurement device 1000 shown in FIG. 10 is a device that forms an image of a measured object 1005, and includes a broadband light source 1001, a mirror 1002, a beam splitter 1003, and a photodetector 1004. A light beam emitted from the broadband light source 1001 is split into a reference light R and a signal light L by the beam splitter 1003. The reference light R travels toward the mirror 1002. The signal light S travels toward the measured object 1005.
As shown in FIG. 10, a traveling direction of the signal light S is defined as the z-direction and a plane orthogonal thereto is defined as the x-y plane. The mirror 1002 can be displaced by a not-shown drive mechanism in the directions of an arrow pointing to both sides in FIG. 10. This is referred to as a z-scan.
The reference light R is subjected to the Doppler frequency shift by the z-scan when reflected by the mirror 1002. On the other hand, the signal light S is reflected by the surface and inner layer of the measured object 1005. The measured object 1005 is a scattering medium, and the reflected light of the signal light S is assumed to have a diffusion wave front with a rough phase including multiple scattering. The signal light S propagated through the measured object 1005 and the reference light R propagated through the mirror 1002 are superimposed by the beam splitter 1003 to become an interference light.
In an image measurement using the OCT technology, only such components of the signal light S that a difference in optical path length between the signal light S and the reference light R is within the μm-order coherent length (coherence length) of the light source and that have a phase correlation with the reference light R, cause interference with the reference light R. That is to say, only coherent signal light components of the signal light S selectively interfere with the reference light R. According to this principle, by executing a z-scan at the position of the mirror 1002 and changing the optical path length of the reference light R, the light reflection profile of the inner layer of the measured object 1005 is measured. Furthermore, it is also possible to scan with the signal light S in the x-y plane direction. By detecting the interference light while executing such scans in the z-direction and in the x-y plane direction and analyzing the detection result (heterodyne signals), it is possible to form a two-dimensional tomographic image of the measured object 1005 (refer to Non-Patent Document 1).
According to the above method, sequential measurements of various sites in the depth direction (z-direction) and tomographic plane direction (x-y plane direction) of the measured object 1005 is required, and therefore, there is a problem with a longer measurement time. In view of the measurement principle, it is difficult to shorten the measurement time.
A method for solving this problem has been also devised. The basic configuration of a device employing this method is shown in FIG. 11. This optical image measurement device 2000 includes a xenon lamp 2001, a mirror 2002, a beam splitter 2003, a two-dimensional photosensor array 2004, and lenses 2006 and 2007. On the light-receiving surface of the two-dimensional photosensor array 2004, a plurality of light-receiving elements are arranged.
A light beam emitted from the light source 2001 is converted into a parallel light flux with a beam diameter expanded by the lenses 2006 and 2007. The beam splitter 2003 splits this parallel light flux into the reference light R and the signal light S. The signal light S is radiated to a range corresponding to the beam diameter. Therefore, the signal light S propagated through the measured object 2005 includes information on the measured object 2005 in the radiated range.
The reference light R and the signal light S are superimposed by the beam splitter 2003 to become an interference light. This interference light has a beam diameter corresponding to the signal light S or the like. The two-dimensional photosensor array 2004 detects the interference light by the two-dimensional light-receiving surface. Based on the detection result, a tomographic image of the measured object 2005 in the abovementioned radiated range is formed. Therefore, it is possible to speedily acquire a tomographic image of the measured object 2005 without scanning with the light beam.
As an example of such a non-scanning optical image measurement device, a device described in Non-Patent Document 2 is known. This device is configured to input a plurality of heterodyne signals outputted from a two-dimensional photosensor array into a plurality of signal processing systems arranged in parallel and detect the amplitude and phase of each of the heterodyne signals.
However, in order to enhance the spatial resolution of an image in such a configuration, it is necessary to increase the number of elements of the array, and furthermore, it is also necessary to install a signal processing system provided with such a number of channels that corresponds to the number of the elements. Therefore, it is difficult to achieve satisfactory resolution in the medical field, industrial field and so on.
Accordingly, the inventors proposed the non-scanning optical image measurement device as described below in Patent Document 1. This optical image measurement device comprises: a light source that emits a light beam; an interference optical system that splits the light beam emitted from the light source into a signal light travelling through a subject placement position where a subject is placed and a reference light travelling along an optical path different from an optical path through the subject placement position, and superimposes the signal light propagated through the subject placement position and the reference light propagated on the different optical path to generate an interference light; a frequency shifter that the interference optical system relatively shifts the frequency of the signal light and the frequency of the reference light; an opto-isolator that splits the interference light into two and periodically interrupts the split interference light to generate two lines of interference light pulses with a phase difference of 90 degrees between the lines so that the interference optical system receive the interference light; photosensors that receive the two lines of interference light pulses, respectively; and a signal processor that the photosensors are spatially arranged, each having a plurality of light-receiving elements that independently obtain light-receiving signals, and a plurality of light-receiving signals obtained by the photosensors are integrated to generate a signal corresponding to each point of interest on a propagation path of the signal light on the surface or in the inner layer of the subject placed on the subject placement position.
This optical image measurement device has a configuration that, in a configuration to split the interference light of the reference light and the signal light into two and receive by the two photosensors (two-dimensional photosensor arrays), the opto-isolator is placed in front of each of the sensor arrays to sample the interference light. Then, the device sets a phase difference of 702 in sampling cycle of the split two interference lights to detect the intensities of the signal light and the reference light composing the background light components of the interference light and the orthogonal components (sin component and cos component) of the phase of the interference light, and subtracts the intensities of the background light components included in the outputs from both the sensor arrays from the outputs of both the sensor arrays to calculate two phase orthogonal components of the interference light, thereby obtaining the amplitude of the interference light by using the calculation result.
As a two-dimensional photosensor array, a commercially available image sensor such as a CCD (Charge-Coupled Device) camera is widely used. However, currently available CCD image sensors have a low frequency response characteristic, and there has been a traditionally recognized problem in which the CCD image sensors are unable to follow a beat frequency of heterodyne signals of about a few KHz to a few MHz. It can be said that the optical image measurement device according to Patent Document 1 devised by the inventors is characterized in that the low response characteristic is utilized to conduct measurements upon full recognition of this problem.
The optical image measurement device as described above has an advantage of being capable of depicting the microstructure of a measured object, for example, acquiring an image at the cell level of a living body, but cannot image the structure of a deep part that a light radiated to the surface of the measured object cannot reach. For example, a conventional optical image measurement device can acquire an image of the fundus oculi, skin tissues and so on of a living body, but cannot image the deep tissue of an internal organ and so on.
In this specification, a site at a depth that a light radiated on the surface of a measured object cannot reach, namely, a site at a depth where an image cannot be acquired when a light is radiated from the surface may be simply referred to as a “deep part.” The depth of the deep part varies depending on a measured object, and also varies depending on the wavelength and intensity of a light.
As a device for imaging the tissues of a deep part of a measured object, an endoscope is known. An endoscope is used to examine the inside of a body by inserting part of the device into an opening (a natural opening or an artificial opening) of the surface of a measured object (for example, refer to Patent Document 2).
An endoscope can thus image the structures of a deep part of a measured object, but cannot depict the microstructure, unlike an optical image measurement device.    [Patent Document 1] Japanese Unexamined Patent Application Publication 2001-330558    [Patent Document 2] Japanese Unexamined Patent Application Publication 2007-125277    [Non-patent Document 1] N. Tanno, “Kogaku”, Vol. 28 Issue 3, 116 (1999)    [Non-patent Document 2] K. P. Chan, M. Yamada, H. Inaba, “Electronics Letters”, Vol. 30, 1753 (1994)